So many things to deal with.
Hard cast is not the best, but pure lead is mostly for paper patched bullets.
20:1 lead:tin is good, antimonial lead can be made to work but keep it simple.
Bullet diameter should be groove diameter at least, better a thou or two over, best at throat diameter.
He needs black powder lube, there are homebrews, but SPG and DGL are good to buy.
405 gr is light but should not tumble regardless. A lot of silhouette shooters will use a lighter bullet for the 200 meter chickens.
Powder charge should be compressed, a little for Swiss, a lot for Goex. I don't know the recommendations for other brands, but some for sure. Done with a compression die, not by cramming a soft lead bullet down with the seating die.
Powder should be settled by loading through a long drop tube even before compression.
The Goex recommendation was to fill the case full of loose powder, and weigh that amount. When loaded with drop tube and compression die so the bullet can be seated al the way down on the wad, with no air space but not squashing the bullet, he has a good starting place. Weigh your loads or use a GOOD measure. Those brass muzzleloader thingies are not accurate enough.
He needs a hard card wad over the powder, I use Walters veg fiber.
Real black is best. I knew one guy who got good accuracy with Pyrodex. One.
Other fakes are not allowed by NRA and not worth fooling with anyhow.
Brass should be sized, expanded, and flared to avoid scraping the lead bullet. Some shooters put on a bit of crimp, I don't.
Starline brass needs annealing before use and it can help others.
The old recommendation for magnum primers with black is out of date. CCI BR or Federal Match are good, there is some use of pistol primers.
Accuracy with BPCR requires fouling control. A blow tube into the chamber throat and four or five breaths through it between shots will keep fouling soft.
Or wipe between shots, a damp patch and a dry patch between shots is a starting place.